Identifying Cast Iron Microstructure Variation Using Acoustic Resonance Techniques

Cast iron foundries have historically utilized ultrasonic methods to determine nodularity levels in ductile iron parts. The foundries have begun to search for an alternative that would provide a test of the whole part, while eliminating part preparation, consumables, rust and plating issues and safety hazards. Progress has been made in the application and performance evaluation of resonant acoustic method as an alternative tool for detecting unacceptable levels of nodularity in ductile iron. This paper covers other aspects of cast iron microstructure, namely graphite shape, graphite size and matrix microstructure. To identify cast iron microstructure variation within a thick section, samples from different locations of a thick section (2″) were studied. Finally, a preliminary study of effect of having casting skin on resonant frequency was also conducted. Step block castings were produced with above-mentioned variables. Rectangular samples for resonant inspection were machined. The cast iron samples were excited by a calibrated hammer, and the resonant frequencies (RF) were recorded with an accelerometer. The RF measurements were normalized to eliminate weight change affect. Finite element analysis (FEA) was done to show the minimum level of shift in RF due to dimensional variability. The design of experiment analysis showed that the variations in graphite shape and matrix microstructure are identifiable. The graphite size variation due to section sizes is detectable to a certain extent, i.e., variation between 5/8″ and 1″. However, the result obtained in this study does not support the use of resonant frequency shift to identify samples from section sizes 1″, 2″ and 4″. The RF response showed that the samples with surface condition (presence of skin) variation can be identified for a known graphite shape.